This invention relates to an ultrasonic transducer, and more particularly to improvements in frequency, efficiency and radiation power characteristics thereof.
Among conventional ultrasonic transducers capable of high-power radiation, well known is the Langevin transducer cramped by bolts (another name: Tonpilz Transducer) which is described in a paper by RALPH S. WOOLLET, entitled "Power Limitations of Sonic Transducers", IEEE Transaction on Sonics and Ultrasonics, October 1968, on page 221.
In this conventional transducer, as shown in FIG. 1, a piezoelectric vibrator (piezoceramic ring) 101 is provided between a front mass 103 and a rear mass 102. These members are firmly cramped together by using a bolt 104 and a nut 105. Although such a transducer is advantageous for high-power radiation because compression bias stress is applied to the piezoelectric vibrator 101 through the bolt and nut, its available frequency bandwidth is narrow, at most 20%, because of its single resonance operation. The characteristic (relationship between the frequency and the relative current) of the transducer operated at a constant voltage in air is shown by broken line in FIG. 2. As shown clearly in FIG. 2, the maximum current flows and the vibrational energy becomes maximum at the resonant frequency f.sub.o. In FIG. 3, the broken line illustrates the relationship between frequency and relative transmitting voltage sensitivity S.sub.s of the above transducer used in water. From FIG. 3 it is understandable that the relative transmitting voltage sensitivity S.sub.s also shows its maximum near the resonant frequency f.sub.o and the available bandwidth is narrow when the transducer is used in water. The actual fractional 6 dB bandwidth available is at most approximately 20%.
In another type of ultrasonic transducer, an acoustic matching plate of a quarter wave length with respect to the resonant frequency is provided at the acoustic radiating side of the piezoceramic vibrator. Such a transducer has a wide-band characteristic and high efficiency. (See IEE Proceedings, Vol. 131, Part F, No. 3, pp. 285-297, June 1984). The matching plate has an impedance between the piezoceramic vibrator and the water. The matching of the acoustic impedance with water is achieved at the specific acoustic impedance (defined as a product of sound velocity and density) of 3.2.times.10.sup.6 to 4.5.times.10.sup.6 MSK Rayls. The material of the matching plate having such specific acoustic impedance is usually comprised of a compound material of an epoxy resin and inorganic fine-grain glass which is equally distributed in the epoxy resin. The desired acoustic impedance can be obtained by adjusting the mixture ratio of the inorganic fine grain. The acoustic matching plate and ceramic vibrator is stuck together with epoxy adhesive, and the acoustic matching plate is therefore likely to become separated from the ceramic vibrator. Furthermore, the acoustic matching plate of the material mentioned above tends to cause deterioration in linearity under high power radiation. Therefore, this type of transducer is limited in use for small or medium power radiation and is not suitable for high power radiation (for example: parametic array).